signaling directs the development of multiple organs and tissues in embryogenesis, and is the causative factor in multiple congenital and adult diseases. Numerous studies suggest the potential for modulating BMP signaling in the treatment of disorders as diverse as kidney disease, pulmonary hypertension, and in medical applications such as orthopedics, endodontics, and tissue engineering. BMP heterodimers are receiving increasing attention recently due to their higher signaling activity to BMP homodimers and their potential in therapeutics. To understand how BMP signaling can generate diverse cellular responses in a myriad of biological contexts and affect disease, as well as how BMP heterodimers can be most effectively used in therapeutics, it is imperative to understand the mechanism by which BMPs signal. A key function of BMP signaling in vertebrate development is to pattern the cells along the dorsoventral (DV) embryonic axis during late blastula and gastrula stages. BMP signaling activity is thought to act as a morphogen, specifying distinct cell types at different activity levels. Dorsally-emanating BMP antagonists are important in generating the gradient of BMP activity with low levels dorsally and highest levels ventrally. In the previous grant period, BMP heterodimers were found to exclusively signal in zebrafish DV patterning, providing an in vivo, physiological assay in vertebrates for BMP heterodimer signaling. The studies here will use this unique in vivo animal model setting to elucidate the mechanism that leads to the exclusive signaling by BMP heterodimers in zebrafish DV patterning. The results are expected to be broadly relevant to BMP heterodimer signaling mechanisms in other biological contexts. A poorly understood mechanism restricts BMP antagonists to dorsal regions during late blastula stages. A new genetic regulator of this process in zebrafish was identified in the past grant period, which when deficient leads to the ventral expansion of BMP antagonists and dorsal midline mesoderm causing the dramatic formation of multiple dorsal axes. The mutant gene encodes the integrator complex subunit 6 (ints6), representing the first loss-of-function mutation of this gene to be studied in any organism. These studies will be extended here to determine how Ints6 functions with the other maternal regulators of DV patterning to elucidate its mechanism of action. Lastly, the molecular nature and role played by a new maternal-effect mutant gene with a defect similar to ints6 will be studied. It is postulated to encode a novel gene or an already known gene with a new function in DV patterning, which will allow the elaboration of the molecular mechanisms mediating vertebrate DV patterning. !